A double hung window is one with upper and lower sashes vertically slidable in a window frame. Such a window usually includes a lock to clamp the upper and lower sashes together when they are closed. The common form of this type of window lock includes a "strike" member mounted on the bottom rail of the upper sash, and a rotatable locking member mounted on the top rail of the lower sash. The locking member is rotatable into and out of locking engagement with the strike member when the sashes are closed. The window is therefore lockable only when it is closed. When it is open, it is unlocked and the sashes are freely movable up and down.
The prior art also includes U.S. Pat. No. 761,568 issued May 31, 1904 to Wedler, disclosing a double hung window with adjustable locking. An extension arm is attached to the upper sash, pivotable from a horizontal (inoperative) position to an erect (operative) position. The extension arm has a series of holes spaced along it. A lever-operated plunger mechanism is attached to the lower sash. The plunger is movable into and out of the extension arm holes to lock the window closed, or at a selected open position. The Wedler lock prevents relative up or down movement of the window sashes, but it is a relatively complex mechanism. Furthermore, it does not pull or tighten the window sashes together against rattle or air leakage.